Trans acting up after changing radiator

Well, the trans cooler line are connected to the radiator. Did you add more trans fluid to the system when you replace the radiator? If not, you are low on fluid causing the shifting issue. Make sure the car is parked on a level surface, and check the fluid level on the dipstick. Engine on and warmed up. Add to the dipstick tube with a funnel , add a little at a time, My guess is around a half a quart.

Why did you change the radiator? If Coolant was mixing inside the transmission, this will also give you problems. You need to fluid the trans and add fresh fluid. But the clutches might already be swollen and your looking at a rebuild or replacement.